Frequency changing circuit arrangement



1952 M. J. o. STRUTT EI'AL 2,532,725

FREQUENCY CHANGING CIRCUIT ARRANGEMENT Filed July 2, 1946 OSCILLATOR 7INVENTORS. MAXWILIAAN JULIU5 OTTO man By ALBERT mNDER ZLEL AGENIPatented Jan. 15, 1952 FREQUENCY CHANGING CIRCUIT ARRANGEMENTMaximiliaan Julius Otto Strutt and Aldert van der Ziel, Eindhoven,Netherlands, assignors to Hartford National Bank and Hartford, Conn., astrustee Trust Company,

Application July 2, 1946, Serial No. 680,964 In the Netherlands May 3,1943 Section 1, Public Law 690, August 8, 1946 Patent expires May 3,1963 6 Claims. (Cl. 250-20) This invention relates to acircuit-arrangement for frequency-changing of electric oscillations bymixing them with locally generated oscillations, which comprises adischarge tube containing a cathode, an input electrode, an outletelectrode and one or more further electrodes having a positive bias, inwhich the (high-frequency) oscillations to be changed in frequencyjointly with the local oscillations are supplied to the said inputelectrode and in which the frequencychanged (intermediate-frequency)oscillations are taken from the said output electrode.

The object of the invention is to provide means which permit a materialreduction of the noise occurring in such circuit-arrangements. Thisnoise, which particularly in the'transmission of feeble signals is asource of great trouble, is due, on the one hand, to spontaneous voltagefluctuations (Brown's movement of the electrons) in the circuitsconnected to the discharge tube and on the other to current fluctuationsin the discharge tube itself. The latter current fluctuations can bedistinguished in emission-fluctuations (irregularities in the flow ofemission of a cathode) and in subdivision-fluctuations (irregularitiesin the subdivision of the current between two or more electrodes havinga positive bias). The noise due to these two latter causes is referredto respectively as the cathode-noise and as the subdivision-noise.

In the case of screen-grid tubes the subdivisionnoise is, in general,considerably stronger than the cathode-noise. It may be observed in thisconnection that the subdivision-noise current in the anode circuit isequal and opposite to the subdivision-noise current in the screen-gridcircuit. This is because an increase of the anode current owing to avariation of the subdivision of the current involves an identicaldecrease of the screen-grid current. The subdivision-noise consequentlybecomes manifest as an alternating current which passes from the anodeto the screen-grid, but which does not occur in that part of the cathodeconductor which is common to the anode circuit and to the screen-gridcircuit. Due to this, the signal-to-noise ratio with screen-grid tubesis materially higher in that part of the cathode conductor than in theanode circuit. 7

..It has been proposed before to make use of this .circumstance toreduce the noise in the output circuit of a frequency-converter circuitby means of a positive (regenerative) feed-back for the frequency rangeoccupied by the (high-frequency) oscillations to be changed infrequency,

the feed-back voltage being taken from the cathode conductor. Thishigh-frequency feedback considerably increases the signal voltageoccurring in the input circuit. A high-frequency noise voltagecorrelated with the cathode noise is introduced at the same time in theinput circuit. This noise voltage after frequency change, leads to anincrease of the intermediatefrequency cathode-noise current passing inthe output circuit. The increase of the cathodenoise current in theoutput circuit in consequence of the feed-back is, however, found oncloser examination to be smaller than the increase of the signalcurrent. In addition, the feed-back does not at all affect thesubdivision-noise current in the output circuit, so that the ratiobetween the signal current and the subdivisionnoise current increasesmaterially. The resulting intermediate-frequency signal-to-noise ratiointhe output circuit is consequently considerably increased by thefeedback; in the case of highly intense feedback this ratio becomesnearly equal to the high-frequency signal-to-noise ratiocathode-conductor.

The circuit-arrangement described hereinbefore has the disadvantage thatthe required strong positive feedback makes it very difi'icult to avoidself-excitation when tuning through a wide frequency-range. It has beenproposed before to obviate this disadvantage by using, at the same time,a negative (degenerative) feedback which compensates the action of thepositive feedback so far as the tendency to self-excitation isconcerned, but which does not disturb the favourable signal-to-noiseratio obtained by means of the positive feedback. The latter requirementis fulfilled if the feedback voltage for the negative feedback is takenfrom the intermediate-frequency output current, because in this case allthe noise components of the output current are reduced by the negativefeedback to the same extent as the signal. However, this again involvesthe disadvantage that thefrequency of the feedback voltage for thenegativefeedback must be reduced to the frequency of the using apositive feedback for the frequency-bandoccupied by theintermediate-frequency oscillain the tions, the feedback current orvoltage being taken from a circuit in which the signal-to-noise ratiofor the said frequency-band is .higherthan in the circuit of the outputelectrode.

For this purpose an oscillatory circuit coupled to the said circuit andtuned to the intermediate frequency may be included in the'circuit ofthe input electrode. The feedback current or voltage is preferably takenfrom that portion of the cathode-conductor which is common tothecircuits of the output electrode and of the .said furtherelectrodets) having a positive bias.

An intermediate-frequency feedback arrangement has, according to theinvention, important advantages over the high-frequencyfeedbac'karrangement proposed before. In; particular, the arrangement, accordingto the invention, permits the employment of very stron feedback withoutdanger of oscillation as the tuning frequency-of the circuit is varied.If the stability is, nevertheless, still insuflicient, or if theselectivity is excessively increased by I the positive 'afeedback, 2a

negativefeedback for the -,frequency-band occupied bytheintermediate-frequency oscillations maybe used at the-same time, thefeedbackcurrent :or voltage for the negative feedback "being taken fromthe circuitof the output electrode. This negative feedback permitseliminating, wholly or. in part, the effect-of the positivefeedback withregard to :the selectivity and the tendency to oscillation, while at thesame time the favourable .signal-to-noise ratio-obtained by means of thepositive feedback .is notidisturbed, because all theintermediate-frequency noisecomponents .in the output circuitare-reduced to the same extent as the signal by :the negative feedback.If the twosfeedbacks .BJBZSEt'lID bythe induction of a feedback voltageinto wan'osc'illatory circuitincluded the circuit "of the: inputelectrode'and tuned to the intermediate-frequency, the negativefeedback: is zpreferabiy aproportioned in such .manner that it fbIiIlgSabout :a damping ;of the-said oscillatory mircu-it which issubstantially equal to the negativerrilampingtproduced by the positivefeedback.

As I has beenpointed out. hereinbeioreptheiprior circuit employingpositive "and nega'tive highfrequency.feedback.r.equired.:meansitoinhangethe frequency of the negative:feedback :voltage :to

the frequency of .the signal. Th8jCiIZCLlit,&0COTding tothe invention,"having-positive and negative 1 intermediate frequency feedback displaysa great advantage over this prior .zcircuit .inithat there is norequirement for changing the frequencyof the negative :feedback voltage.

.Aszmay be assumed'to' bea'known, the conversion conductance; that isto. say itlieintermediatefrequencytsignal currentin:thercircuitofrtheoutput eelectm'dexper 'volt ofJhigh-frequency signal voltage atttheinput .electrodais highly dependent-uponxthe amplitude of :the .localoscillations supplied'to the tube. :It. is thus found that if ahigh-frequency positive feedback is used a maximum signal-to-noiseratiois achieved in the output impedance'if theconversion conductance ischosenzto be low. Forthis reason it has been proposed before to choosethe amplitude of the local oscillations and the operating voltages ofthedischarge tube in the case of afrequencychanging circuit havingia;high-frequency positive feedback, in such a manner thattheeonversionconductance amounts at the .most to ,one fifth of the mutual conductanceof :the discharge tube for the high-frequency oscillations to bechanged. This arrangement, however, does vnot 4 apply to thecircuit-arrangement according to the invention in which anintermediate-frequency positive .ieedbackis-used an'din which theamplitude of the local oscillations should, preferably, be such that theconversion conductance has, at least approximately, a maximum value.

In order that the invention may be clearly understood and readilycarried into effect, it will now beset out more .fully with reference tothe accompanying. drawin in which one embodiment of the invention is,illustrated by way of example.

In .the frequency-changing circuit shown on the drawing thehigh-frequency oscillations received by an aerial I are transmittedinductively to'an input oscillatory circuit 2, which in serieswith-ac0il3coupled to a local oscillator 4 (shown diagrammatically) isincluded in the input cir- .nected to ground in the usual manner viaithe parallel combination of a resistance .14 :and .:-a condenser l5,so'that a suitable negativebias for the control-grid -l .is obtained.'Apositive zbia-s is supplied to the'screenegrid 8 and'to thepanode 9via resistances It .and H. Jointly with the source of voltage theseresistances .are shunted by condensers l8 and I9 "which constitute ashort-circuit both;for theyhigh-frequency. -and:forthepintermediate+frequencywoscillations.

According to .the invention, the commonpart of the connections 'of.the-cathodeifi to therscreengrid '8 andto :the anode .Qincludesaninductance 20 whichiis coupled .toxan oscillatory :circuit 2] which istuned :to the .intermediatesfrequency and which in .series with :the;circuit .2 :and the coil 3 .is :included .in the :circuit cf :the:controlgrid 1. .Thesense :of'the coupling between the coil 20 and the"circuitll I is 'suchthatapositive feedback for theintermediate-frequency :pscilla tions is obtained. This feedback'has the.effect of materially increasin the signal-voltage :a-croze;s the outputcircuit Hl. The*sub'division-noisepurrent, which is set up byfluctuations ofthe subdivision of the currentbetweenthe screen-grid 8and the anode 9, does not "pass, however, through the coil '20 *and isconsequently not biassed by the feedback. This results in'a-materialincrease of the signal to-noise'ratio.

It should be--noted, however, that the-addition of the circuit ZI -byitself causes an increase of the noise-current intheano'de circuit inconsequence of the spontaneous potential fluc'tuation's occurring inthis circuit. Ifthis-increase were of the same order of 'magnitude'as'theisub'division-noise current, the use of apos'itive feedback wouldin practice no longer be effective. the value of the said voltage'fluctuations'isi'determined by the impedance of the circuit2l itistherefore desirable that this impedance should not be excessively high.A suitable value is that of 800 ohms, for instance.

The positive feedback by means of the coil lfl also gives-rise toanincrease df=the-selectivity of the circuit; this increase may beundesirable under certain conditions. .In. addition, :the positivefeed-back may have .an unfavourable effect in some cases on thestability :of the :circu-it.

, These disadvantages can-bephviatedibxalsozus- Theintermediate-frequency .oscil- 5.1 mg a"ne'gative feedback; To this end,the anode circuit of the circuit-arrangement shown in the drawingincludes a coil- 22 which is coupled to the circuit 2! in such sense asto secure a negaf-i tive feedback for the intermediate-frequencyoscillations' This negative feedback has again the effect of'reducingthe output voltage occurring across the circuit [0. However, as the coil1'2 is included in the connection between thescreen-grid '8 and theanode 9, so that the sub-'5" division-noise currentpasses through thecoil' 22, the subdivision-noise voltage is reduced to the same extent asthe signal voltage, so that the favourable signal-to-noise ratioobtained by meansofrthe positive feedback is'not disturbed.

In the embodiment of the invention hereinbefore described a tetrode isused to constitute the mixing tube.v If desired, a tube having moreelectrodes,.for instance a pentode or a hexo'de may be used. It is,however, essential in connec tion with the efi'ect aimed at thatthe(high; frequency) oscillations to be changed and the localoscillationsshould be supplied to the same not any intermediate-frequency currentpassing;

through it, sothat the feedback by means of the coil 20 would have noeifect..

What we claim is: q

1.? An electric circuitarrangement for mixing a first wave and a secondwave, comprising. an electron discharge tube having cathode, controlgrid, accelerating and anode electrodes, first means intercoupling saidcontrol .grid and cathode electrodes and constituting a controlgridcathode circuit, second means intercoupling said accelerating andcathode electrodes and constituting an accelerating electrode-cathodecircuit, third means intercoupling said anode and cathode electrodes andconstituting an anode-cathode circuit, an output circuit coupled to saidanode, means to apply said first and second waves to said control gridto develop in said output circuit an intermediate frequency wave, animpedance element connected in common to said anode-cathode and saidaccelerating elec- I trode-cathode circuits, and means to couples'aidimpedance element to said control grid-cathode circuit in regenerativerelationship at saidintermediate frequency to apply said intermediatefrequency wave to said control grid.

2. An electric circuit arrangement for mixing a first wave and a secondwave, comprising an electron discharge tube having cathode, controlgrid, accelerating and anode electrodes, first means intercoupling saidcontrol grid and cathode electrodes and constituting a controlgridcathode circuit, second means intercouplingsaid accelerating andcathode electrodes and constituting an accelerating electrode-cathodecircuit, third means intercoupling said anode and cathode electrodes andconstituting an anode-cathode circuit, an output circuit coupled to saidanode, means to apply said first and second waves to said control gridto develop in-.; said output circuit an intermediate frequency wave, anoscillatory circuit coupled in said control grid cathode circuit andtuned to the frequency of said intermediate frequency wave, an impedanceelement connected in common to said anodecathode and said acceleratingelectrode-cathode 6.. circuits, and means to couple said impedanceelement to said oscillatory circuit'in regenerative relationship at saidintermediate frequency to apply said intermediatefrequency wave to saidcontrol grid. 1

3. An electric circuit arrangement for mixing a first wave and'a secondwave, comprising an electron discharge tube having cathode, controlgrid, accelerating and anode electrodes, first means intercoupling saidcontrol grid and cath-' odeelectrodes and constituting a controlgridcathode circuit, second means intercoupling said acceleratingandcathode electrodes and constitutingan accelerating electrode-cathodecircuit, third means intercoupling said anode and bathode electrodesandconstituting an anode-cathode circuit, an output circuit coupled tosaid anode, means to apply said first and second waves'to said controlgrid to develop in said output circuit an intermediate frequency wave,-alow'impedance oscillatory circuit coupled in saidcontrol'grid-cathode'circuit and tuned to the frequency'of saidintermediate frequency wave, an impedance element connected in common tosaidanode-cathode and said acceleratingelectrode-cathode circuits, andmeans to couple said impedance element to said oscillatory circuit inregenerative relationship at said intermediate frequency to apply saidintermediate frequency wave to said control grid.

4. An electric circuit arrangement for mixing a first wave and a secondwave, comprising an electron discharge tube having cathode; controlgrid, accelerating and anode electrodes, first means intercoupling saidcontrol grid and cathode electrodes and constituting a controlgridcathode circuit, second means intercoupling said accelerating andcathode electrodes and constituting an accelerating electrode-cathodecircuit, third means intercoupling said anode and cathode electrodes andconstituting an anode-cathode circuit, an output circuit coupled to saidanode, means to apply said first and second waves to said control gridto develop in said output circuit an intermediate frequency wave, afirst impedance element connected in common to said anode-cathode andsaid accelerating electrodecathode circuits, means to couple said firstimpedance element to said control grid-cathode circuit in regenerativerelationship at said intermediate frequency to apply said intermediatefrequency wave to said control grid in positive feedback relationship, asecond impedance element coupled in said anode-cathode circuit, andmeans to couple said second impedance element to said controlgrid-cathode circuit in degenerative relationship at said intermediatefrequency to apply said intermediate frequency wave to said control gridin negative feedback relationship.

5. An electric circuit arrangement for mixing a first wave and a secondwave, comprising an electron discharge tube having cathode, controlgrid, accelerating and anode electrodes, first means intercoupling saidcontrol grid and cathode electrodes and constituting a controlgridcathode circuit, second means intercoupling said accelerating andcathode electrodes and constituting an accelerating electrode-cathodecircuit, third means intercoupling said anode and cathode electrodes andconstituting an anode-cath ode circuit, an output circuit coupled tosaid anode, means to apply said first and second waves to said controlgrid to develop in said output circuit an intermediate frequency wave,an oscillatory circuit coupled in said control grid- 7 cathode :circuit.and tuned :to the :frequency :of saidfintermediatelfrequency wave, afirst impedance element. connected in common to saidanodecathode'circuit and, said accelerating electrode-cathode circuit,means to inductively couple said first impedance elementtosaidoscillatory circuit in regenerative relationship at saidintermediate frequency toapply said intermediate frequency wave to saidcontrol gridin positive feedback relationship, a second impedanceelement coupled in said anode-cathode circuit, and means lie-inductivelycouple said second impedance element to said oscillatorycircuitlin'degenerative relationship at said intermediatefrequency toapply said intermediatafrequency wave tosaid control grid negativefeedback relationship. V

r electriccircuit arrangement for mixing a first wave and a second wave,comprising an electron discharge tube having cathode, control,accelerating 'and' anode electrodes, first means intercoupling saidcontrol grid-and cathode-electrodes :and "constituting a controlgrid-cathode circuit, second means intercoupling said accelcrating andcathode electrodes and constituting an accelerating electrode-cathodecircuit, third means intercoupling said anode and cathode electrodes andconstituting an anode-cathode circuit,an output circuit coupled :tosaid-anode,

means to apply said first and second'wave to said control-grid todevelop inisaid output circuit an intermediate frequency wave, anoscillatory circuit coupledin said control grid-cathodecircuit and:tuned'to the frequency of :said:intermediate: frequency'wave, a firstinductive (element connected in common to'saidanode-cathode circuit and'said accelerating electrode-cathode circuit, a second'inductive elementcoupledin said anode-cathode circuit, means to inductively couplesaidsecond inductive element'to said oscillatory circuit in degenerativerelationshipat said intermediate frequency to'provide "a given dampingforsaid oscillatory circuit, :and means to :inductively couple saidfirst inductive elementto said oscillatory circuit in "regenerativerelationship at said intermediate frequency thereby to counteract saidgiven damping.

'MAXIMILIAAN JULIUS OTTO 'S'I'RU'ITJ ALDERT VAN D R ZIEL.

REFERENCES CITED The following references 'are'of record in'the'file'o'f this patent:

' UNITED STATES PATENTS Great Britain 'Nov. 16, 1931

